GB2070332A

GB2070332A - Control of photodector system

Info

Publication number: GB2070332A
Application number: GB8104947A
Authority: GB
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1980-02-22
Filing date: 1981-02-17
Publication date: 1981-09-03
Also published as: JPH0449054B2; FR2476916A1; FR2476916B1; DE3105910A1; DE3105910C2; JPS56133628A; GB2070332B; CA1164555A; US4321486A

Description

1 GB 2 070 332 A 1

SPECIFICATION

Electromagnetic radiation sensor and processor device The present invention relates to the control of photodetector signals, particularly signals from photodetectors which are used s sensors in receiving radiation passing through some portion of an imaging system. More particularly, the signals controlled in the present invention are specially connected with sensors for sampling electromagnetic radiation transmitted through the system optics.

The use of charge-transfer device technology to process signals obtained from photodetectors has a number of advantages. First, charge-transfer devices, particularly charge- coupled devices, can be relatively 10 easily fabricated in silicon monolithic integrated circuits, and can be fabricated such that these devices individually are capable of being provided therein in a high density. The charge-transfer device with respect to analog signals, is basically a sampling device directly manipulating the analog samples. Thus, interface between such a device and the photodetector can be relatively uncomplicated since the photodetector, typically, provides an analog electrical output signal more or less related to that electromagnetic energy which has been sensed therein. Further, the analog samples to be manipulated in the charge-transfer device can be controlled by digital clocking circuits which permit considerable flexibility in treating the analog samples.

If charge-coupled device technology is to be used for processing the signals resulting from the photodetector, a very convenient photodetector to use is a conductor- insulator-semiconductor (CIS) detector 20 which is essentially a capacitor. In such photodetectors, the semiconductor material supports an insulator which in turn supports the conductor, the side of the conductor opposite its insulator support being first exposed to impinging electromagnetic radiation of interest for detection. The conductor is an electrode which together with the insulator are of such a nature as to permit radiation to pass therethrough to reach the surface of the semiconductor material. With the voltage applied between the conductor serving as a photodetector electrode and the semiconductor material such as to form a depletion region in the semiconductor material, charges accumulate at the surface of the semiconductor material at the sensing site in proportion to the amount of radiation experienced at that semiconductor material surface. This radiation induced charge is accumulated and held at the semiconductor material surface at the sensing site for the time duration that voltage is maintained on the conductor because of the potential resulting at the semiconductor material surface due to this applied voltage. In a typical photodetector system, the conductor voltage is typically a repeated voltage pulse changing between zero voltage and some applied voltage level, as just indicated, with such a voltage pulse being provided to the conductor at each sensing site. Thereby, sampling is accomplished of the incoming radiation at various points across a phase surface thereof in the sampled data photodetector system.

However, there is a maximum amount of radiation induced charge accumulation that is desired at a sensing site, either because (i) the photocletector cannot accumulate any further charge at the sensing site for the voltage present there, or because, (ii) the charge-coupled device signal processing circuitry design can operate only with a certain maximum amount of accumulated charge in a period of time, i.e. a maximum size charge packet representing a sample in a sampling period. One possibility, then, would be to provide a 40 fixed time duration for accumulating charge induced by radiation impinging at a sensing site in a sampling period. Such a procedure cannot always be relied on to prevent an overaccumulation of charge during a sampling period at a sensing site. This is because the intensity of the radiation impinging on the sensing site will often be unknown because the scene being imaged will usually have a substantial variety of radiation intensities thereacross which in many instances cannot be predicted, either as to the absolute intensity maximum that will occur or as to the locations of intensity maxima in the scene. Thus, the maximum amount of charge which will accumulate in a fixed time duration in a sample cannot always be predicted either, nor can the particular sensing site be predicted at which such a maximum sample will occur.

- Another possibility would be to transfer the accumulated charge, or sample, from each sensing site into a sequential position in a charge-coupled device shift register and monitor the size of each charge packet as is transferred by a selected monitoring point. Then, the time duration could be varied depending on the size of the charge packets coming by the monitoring point. However, the result is that the charge packets do not have a maximum size selected for them during the sampling period the packet is being accumulated raising the possibility that too large a charge accumulation will occur during that sampling period before the monitor senses the situation. Further, the monitoring process takes additional time which can interefere with the photodetector signal processing insofar as limiting the rate of changes which can be sensed in the scene being imaged because of the limited time response of the photodetector signal processing system.

Thus, a photodetector signal system would be desirable in which the size of the charge packets occurring atthe various sensing sites is controlled at the very time during which these charges are being accumulated.

Further desired would be a photodetector signal processing system in which the charges accumulating in the packets of each sensing site would be simultaneously monitored so that the size of the charge packets at every sensing site in the array would affect the decision as to when sensing should be terminated during any particular sampling period, i.e. when the sensing portion should terminate in a particular sample period or frame.

According to the invention, there is provided an electromagnetic radiation sensor and processor device 65 2 GB 2 070 332 A 2 having a plurality of sensing sites; said device comprising a semiconductor material body of a first conductivity type except in selected regions thereof, a plurality of input charge-transfer devices each of which has therein a storage well electrode, and each said input charge- transfer device storage well electrode being spaced apart from a major surface of said body by an electrical insulating layer; a sensor electrode having therein a plurality of separated regions each region being immediately adjacent to a respective one of said storage well electrodes such that transfers of electrical charge can be directed therebetween, said electrode being spaced apart from said major surface by an electrical insulating layer, said first sensor electrode regions being located at at least some of said electromagnetic radiation sensing sites; switching means having first and second terminating regions and having a control region therein by which said first switching means is capable of being directed to effectively provide a conductive path of a selected conductivity between said first and second terminating regions, said first terminating region being electrically connected to a first terminal electrical connectable to a first electrical energization means, said second terminating region being electrically connected to said first sensor electrode; and a buffer having a high impedance input electrically connected to said first sensor electrode, said buffer being capable of providing a representation at an output thereof of any voltage occurring at said input thereof whereby an 15 indication of voltage occurring on said first sensor electrode can be provided in response to selected electromagnetic radiation impingement occurring therethrough at said electromagnetic radiation sensing sites.

According to the invention, there is also provided a method of sensing electromagnetic radiation for deriving signals representative of the sensed radiation, the sensing device having a semiconductor material 20 body of a first conductivity type except in selected regions thereof with said semiconductor material body having a major surface, said device having a sensor electrode adjacent said major surface at least at a plurality of electromagnetic radiation sensing sites, said device also having a plurality of detector signal transfer devices arrayed along said sensor electrode such that there is a detector signal transfer device immediately adjacent to each of said plurality of electromagnetic radiation sites, said method comprising 25 providing for a first selected duration, a voltage of a selected value between said sensor electrode and at least that portion of said semiconductor material body at said plurality of electromagnetic radiation sites and subsequently, for a selected second duration, eliminating any significant conductive paths connected to said sensor electrode; monitoring those values of voltage occurring between said electrode sensor and said semiconductor material body during said second selected duration while said electromagnetic radiation is 30 impinging on said sensing sites; and in sequence, repeating each of said preceding steps a selected number of times beginning with providing a voltage of said selected value between said sensor electrode and said semiconductor material body for a said first duration afterthat immediately preceding said second duration is past.

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings in which:- Figure 1 shows a cross-section of a monolithic integrated circuit of an electromagnetic signal sensing and processing device of the present invention, Figure 2 shows an equivalent circuit schematic diagram of a larger portion of the monolithic circuit which includes that which is shown in Figure 1, Figure 3 is a graph showing selected performance characteristics of the circuit diagrammed in Figure 2, Figure 4 is a schematic diagram of the electromagnetic signal sensing and processing device of the present invention, and Figure 5 is a graph showing several signals occurring in the device of Figure 4.

Referring to the drawings, Figure 1 shows a cross section of a monolithic integrated circuit at a location therein where an array of conductor-insulator-semiconductor (CIS) photodetectors are fabricated. In this array, there is a plurality of CIS photodetectors, provided having a common conductor through which electromagnetic radiation can pass at the photodetector site to reach the semiconductor material below after passage through the insulating layer separating this conductor and the semiconductor material. The cross section of Figure 1 is 50 taken along a portion of this interconnected group of CIS photodetectors, more particularly, taken through the common conductor itself and the structure immediately therebelow.

A monolithic integrated circuit is formed in and on doped silicon serving as the semiconductor material base which is designated in Figure 1 by the numeral 10. This base body of the semiconductor material, except for possible selected regions therein, is doped to have a p-type conductivity by the presence of boron 55 atoms. The conductivity typically in the range of 9 to 13 Q-crn or approximately 1 x 1015 boron atoms. cm3.

The major surface, 11, of semiconductor material body 10 has formed thereon an insulating layer, 12.

Insulating layer 12 is comprised of silicon dioxide and varies in width along major surface 11. Thus, in the field regions of the device, surrounding the photodetecting sites, the corresponding insulator layer portions are designated 12'while the feature portions of the device at which the photocletecting sites occur have the 60 corresponding portion of insulating layer 12 designated 12". The thickness of the insulating layer in field region portions 12' is on the order of 6,700 A while the thickness of the insulating layer in feature region portions 12" is on the order of 1,100 A.

Insulating layer 12 also has a major surface, 13, upon which a doped polycrystalline silicon conductor, 14, is provided which serves as the electrode which is common to each of the CIS photodetectors, the electrode 65 1 i 3 GB 2 070 332 A 3 through which radiation to be sensed by the photocletectors passes at the sensing sites as earlier mentioned.

The polycrystalline silicon forming electrode 14 is doped with phosphorus in sufficient concentration to lead to a sheet resistivity of 15 to 50 Q/Mto render the polycrystalline silicon in electrode 14 conductive. Electrode 14 is 5000 A thick.

Electrode 14 has a major surface, 15, upon which is formed a passivating insulating layer, 16. Layer 16 is 5 also formed of silicon dioxide and has a thickness of 4000 A.

Finally, layer 16 has a major surface, 17, upon which is formed an electromagnetic radiation barrier, 18, the barrier being aluminum which is impervious to light. The thickness of barrier 18 is 1.2[tm. Barrier 18 has openings at the photodetecting sites in the feature region of the device to permit light to pass through insulating layers 12" and 16 as well as through electrode 14 to impinge on semiconductor material 10 at 10 these sites.

Also, to assure that there is no significant interaction because of voltage being applied to electrode 14 with semiconductor material 10 in the field regions, considerably higher conductivity regions, 19', are provided in semiconductor material 10 immediately below insulating layer regions 12'. These regions are doped also with boron to a concentration of 1 X 1018 atoMS/CM3.

Finally, to influence the characteristics of the CID photodetectors, slightly higher conductivity regions, 19", are formed in semiconductor material 10 immediately below insulating layer regions 12". These regions are shown by short, diagonal lines in Figure 1 with these regions being doped by boron atoms in a concentration of 2 x 1015 ato MS/CM3. This leadsto an inversion threshold atthe sensing sites of approximately 0.5 volts.

The structure and the method of operation of the circuitshown in Figure 1 leadsto certain effective capacitances being experienced when the structure of Figure 1 is operated in an electrical circuit. In typical operation, a voltage is applied between electrode 14 and semiconductor material body 10 which causes depletion regions, 20, to form in semiconductor material 10 atthe photodetecting sites in the feature regions.

These depletion regions are outlined by major surface 11 of semiconductor material 10 and by long dashed lines at the photocletecting sites in semiconductor material 10 which intersect major surface 11. Such 25 depletion regions do notform underthe field regions because of the substantially greater thickness of insulating regions 12' and because of the higher conductivity regions 19'. The value of the voltage applied to electrode 14 might typically be about 3.5 volts.

With this voltage applied to electrode 14, the impingement of electromagnetic radiation on the photodetecting sites leads to the accumulation of charge carriers, in this case electrons, in semiconductor 30 material 10 at major surface 11 in the feature regions and within the depletion regions 20. Electromagnetic radiation in the form of light is represented in Figure 1 by a horizontal series of vertical arrows directed toward the structure at the photocletecting sites and at the adjacent portions in the field regions. The electromagnetic radiation impinging above the field regions is blocked from reaching any further into the structure of Figure 1 by barrier 18. Thus, the applied voltage establishes a potential well at surface 11 in the 35 feature region photodetecting sites into which electrons, generated by impinging radiation, are captured.

The addition of the radiation induced electrons at the photodetecting sites as charge carriers reduces the depth of the potential well.

The accumulated radiation induced electrons or charge carriers form a charge packet in semiconductor material 10 at surface 11 in each photocletecting sites, this packet carrying, in the amount of charge therein, 40 the information as to how much radiation has passed through the opening in barrier 18 and reached semiconductor material 10. The greater the intensity of the radiation reaching semiconductor material 10 at a photodetector site, the greater the amount of charge captured in the potential well in semiconducting material 10 at a site in a given amount of time. Again, this potential well is due to the resulting surface potential occurring at major surface 11 in depletion regions 20 in the feature region photodetecting sites, this 45 surface potential being represented by sites by (s which depends on both electrode voltage 14, even though perhaps due at times to only the charge on the capacitances effectively connected thereto, and on the amount of accumulated radiation induced charge.

This situation at each photocletecting site can be represented by an equivalent capacitance, the depletion 5() region capacitance, which is variable in value with the amount of voltage applied between electrode 14 and 50 semiconductor body 10 and by the amount of radiation induced charge accumulated at major surface 11 in depletion region 20 in the feature region photodetecting sites. This depletion region capacitance is shown in dashed line form across depletion regions 20 at each photodetecting site in Figure 1 and is designed by c,1.

The other capacitances present in the structure shown in Figure 1 are only structure related and are not significantly affected by the applied voltage and the accumulated radiation induced charge carriers. These are the capacitances occurring between electrode 14 on one side of insulating layer 12 and semiconductor material 10 on the other side. In the feature region, such a capacitance occurs in series with the depletion region capacitance just discussed. This capacitance, designated c,,,,p, is a capacitance occurring at photodetecting sites in the feature regions based on an oxide dielectric because of the presence of region 12" separating conductor 14 and semiconductor material body 10. The corresponding capacitance in the field regions, designated c,,,,f, arises because of insulating region 12'separating electrode 14 and semiconductor material body 10. Capacitance coxp is in series with capacitance Cd which together are in parallel with the adjacent capacitance c.xf because of the common interconnection of the capacitances in series with c.xf by electrode 14 and semiconductor material 10. Due to the closely uniform results that come about from the fabrication methods used in manufacturing monolithic integrated circuits, these capacitances at each feature 65 4 GB 2 070 332 A 4 region photodetecting site and at each field region will be substantially equal in value to similar capacitances formed in the repeated, symmetrical structures occurring along the array portion having electrode 14 as a common electrode. Thus, each of these equivalent capacitances having the same subscript are substantially equal to one another.

The equivalent capacitances of Figure 1 can be viewed as part of an equivalent circuit as shown in Figure 2.

Ratherthan having just two feature region photocletecting sites and all or portions of the three field regions shown in Figure 1, the equivalent circuit of Figure 2 assumes there will be several more photodetectors sharing common electrode 14. Thus, interconnection 14 in Figure 2, which is equivalent to electrode 14 in Figure I in the equivalent circuit, is shown in a manner to indicate that there is a total of N combinations of a feature region photodetecting site and allocated adjacent field regions in the full structure of which a part is shown in Figure 1. That is, the equivalent capacitances for N such combinations are shown each connected to electrode 14 in Figure 2.

The sides of the capacitances, which in Figure 1 are shown connectcl to semiconductor material body 10, are indicated to be connected to ground in Figure 2 assuming that semiconductor material body 10 of Figure 1 is operated at the ground reference potential. Vertical dashed lines are used in Figure 2 to set off each combination of feature region photodetecting site capacitance and associated allocated field region capacitance. The same capacitance designations are used in Figure 2 as are used in Figure 1 with the addition of a further number representation subscript. This last subscript is to indicate the position along electrode 14 of the particular combination of feature region photocletecting site capacitence and associated allocated field region capacitance.

Further shown in Figure 2 connected to electrode 14 is the source of an enhancement mode, n-channel, insulated-gate field-effect transistor OGFET), 25, which might be a metal- oxide-semiconductor (MOSFET), which has its drain connected to a reference voltage, VREF, and its gate connected to a source of an operation directing signal, (p, This arrangement permits a voltage to be provided at electrode 14 with respect to ground for establishing the depletion region at the feature region photodeteGting sites. Voltage signal 0, is a clock signal which establishes the beginning of a sampling period during which the photodetecting sites connected to electrode 14 are directed to effectively sample the electromagnetic radiation impinging thereat.

A further enhancement mode, n-channel IGFET, 26, is shown in Figure 2 to have the drain thereof connected to electrode 14 and the source thereof connected to ground. A control signal, 0,1 is provided to the gate of MOSFET 26 on those occasions when it is desired to have all of the photodetectors connected to electrode 14 rendered inoperative as transistor 26 can electrically connect electrode 14 to ground.

At the other end of electrode 14 there is shown connected in Figure 2 a further enhancement mode, n-channel IGFET, 27, having its gate connected to electrode 14. The drain of transistor 27 is connected to a supply voltage, Vs, while the source of transistor 27 is connected to a current source load, 28. The other side of current source 28 is connected to ground. Current source 28, for instance, may be formed by a resistance 35 or by another IGFET.

An output voltage, V, is supplied at the source of transistor 27. Output voltage v,, will be shown in the following to provide an indication of the amount of radiation induced charge being accumulated at the various photodetector sites represented in Figure 2. This will be shown for circumstances in which the voltage applied to electrode 14 through MOSFET 25 is only a voltage pulse during a frame, of a duration long 40 enough to change the capacitances connected to electrode 14 but terminated during the actual taking of a sample in the frame or sampling period.

Also shown in Figure 2 is the surface potential at each feature region photodetecting site as was shown to result in the discussion of the structure of Figure 1. These surface potentials were indicated above by (i)s and will be so designated in Figure 2 with the addition of a number representation subscript, i.e. ol, to indicate 45 to which combination in Figure 2 the surface potential representation pertains. In the typical design for the photocletecting sites in Figure 1, the surface potential q), will in general be large compared to the change in the surface potential,L%, due to radiation induced charge accumulating at the photodetecting site during the taking of a sample of the impinging radiation. Thus, to a first approximation the depletion capacitance c,i can be viewed as being constant as it depends on the whole of the surface potential q),, present.

If the situation is considered first where the electromagnetic radiation impinging on each of the detectors is assumed to be identical, the various feature region capacitance branches connected between electrode 14 and ground in Figure 2 can all be considered in parallel and to be identical so that they may be reduced to a single branch connected between electrode 14 and ground. Similarly, a single branch between electrode 14 and ground can be considered to represent a reduction of all of the field region capacitances. This reduced 55 field region branch would have a single capacitance with value equal to direct sum of the values of all of the capacitances Coxfn from 1 to N. The other reduced branch, the feature region branch, would have a capacitance of a value equal to the direct sum of the values of all of the capacitances Cwmn from 1 to N in series with another capacitance having a value equal to the direct sum of the values of all of the capacitances Cdn from 1 to N. The feature region capacitances can be reduced to this reduced branch form because the 60 surface potential at each capacitance juncture between a capacitance coxp and a capacitance c,,,, from 1 to N will be the same due to the assumption of uniform electromagnetic radiation impingement on the detectors.

g, li GB 2 070 332 A 5 Then, using the well known relationship that the charge on a capacitance equals the value of the capacitance times the voltage on that capacitance, the following small signal equation can be written as a matter of conservation of charge:

A,Ceq(reduced(p, node) 24 AV14 Ceq(reduced electrode 14 node), That is to say, the change in the charge across a capacitance connected to one node in the reduced branch circuit must equal the change in charge across the capacitance connected to the other node in the reduced branch circuit.

The above relationship can be rewritten in the following mannerto provide the change in voltage on 10 electrode 14 as a function of the change in surface potential as follows:

IV14 Ceq(reducedp. node) deq(reduced electrode 14 node) The values for the equivalent capacitances occurring at each of the two nodes in the reduced branch circuit can be found from circuit theory to be Ic lc. 20 Cecl(reduced 4% node) n oxpn n xfn + Ic I:C + Ic n dn' n oxpnn xfn 25 IC IC n oxpn n dn C.q(reduced electrode 14 nouei 1C. n xfn Ic + Ec n oxpnn dn Dropping the constraint of equal surface potential changes each photodetector by virtue of the assumption of uniform electromagnetic radiation impinging on them all, linear circuit theory, with its superposition principle, and the second equation set out above allows the conclusion that the change in voltage occurring on electrode 14 because of radiation impinging on the photodetectors will be equal to the sum of the individual changes in the surface potential at each photodetector weighted by the factor K, as defined above, a result which can be written LV14 = KEA45sn. n The change in surface potential at any particular detector is equal to the radiation induced charge developed 40 there divided by the equivalent capacitance occurring at the photodetector. Thus, the change in surface potential at any particular photodetector can be written as follows:

A4),n Qn qN(radindelectrons)n Ceq(individual,p, node)n C,q(individualp, node)n' where q is equal to the electronic charge and N(rad ind electrons)n is equal to the number of electrons induced by the radiation impinging on photodetector n. The equivalent capacitance occurring at a photodetector node n can be found from circuit theory to be the equivalent of capacitance Cdn in parallel with 50 the series combination Of Coxpn and all of the other capacitance branches connected to electrode 14 in Figure 2 taken in parallel, or C,,q(individual;,. node)n Coxpn [Coxfn + (N 1)Clq(reduced electrode 14 node)] + Cdn Coxpn + Coxfn + (N - 1 W,q(reduced electrode 14 node) As a result, the change in voltage on electrode 14 due to radiation impinging on the photodetectors 6 GB 2 070 332 A 6 interconnected thereby becomes, assuming the equivalent capacitances at each individual node are equal because of uniform processing, the following:

K LV14 Mn.

C.q(individual % node) n As can be seen from the equation of this last result, the total change in the voltage on electrode 14, due to radiationinduced charge accumulating in the various photodetectors, is a function of the average charge being accumulated in each photodetector due to radiation impinging thereon, i.e. the size of the charge packet being accumulated, times a constant.

While the foregoing is a somewhat approximate analysis, the results indicate that the average of the size of the accumulating charge packets due to impinging radiation can be successfully determined by monitoring the voltage appearing on electrode 14 in Figure 2 after the capacitances tied to electrode 14 have been charged to a selected value through transistor 25. At the termination of this charging of the capacitances connected to electrode 14 through transistor 25, resulting in a voltage on electrode 14 equal to VREF the voltage on electrode 14 will decay as charge accumulates in the photocletectors due to impinging radiation.

This decay can be noted by observing voltage V. at the output of transistor 27 driving current source 28 in a source follower configuration. When voltage V. has decreased to a sufficiently small value, the observer of this voltage knows that the average size of the charge packets accumulating under the photodetectors connected to electrode 14 has increased to a sufficiently large value indicating that the current taking of a sample in the current frame should be terminated.

This can be seen in Figure 3 where V. has been plotted as a function of sampling period time. Thus, when the capacitances connected to electrode 14 have been charged from voltage source VREF through transistor 25 so that the output voltage V,, appears to equal VREF - VTHRESH-27 and transistor 25 is switched into the "off" 25 condition, the voltage on electrode 14 will begin to decay as will be reflected in output voltage V0. As shown in Figure 3, higher intensities of electromagnetic radiation impinging on the photocletectors connected to electrode 14 will lead to a relatively short period of time for the voltage on electrode 14 to reach a selected voltage level represented by a dashed line in Figure 3. Conversely, lower intensities of electro-magnetic radiation will lead to longertimes before the voltage in electrode 14 reaches the same dashed-line represented voltage level. Thus, by selecting a particular voltage level as representing the desired maximum size of the average charge packet accumulating in the photocletectors connected to electrode 14, the sampling time in a frame will be determined at which point sampling is to be terminated. Thereafter, the accumulated charge packets are to be transferred out of the photocletectors where accumulated for further processing and the photodetectors prepared for a new sampling of the impinging electromagnetic radiation- 35 The sensing and processing device is shown in schematic form in Figure 4, and components in Figure 4 corresponding to those shown in Figure 2 have been similarly designated as they were in Figure 2. Thus, to the left in Figure 4 appear again transistors 25 and 26 connected to electrode 14. Electrode 14, shown in Figure 4 as doped polycrystalline silicon of varying width, connects and forms part of photodetectors 1 through N where each of these photodetectors represents an enlarging at its site of electrode 14, i.e. over the 40 feature region. Electrode 14 narrows to form the interconnection portion between the photodetector sites, i.e. over the field region.

To the right in the system of Figure 4, electrode 14 is connected to the gate of transistor 27, just as in Figure 2. Again, transistor 27 drives current source 28.

Transistors 25 through 27 are shown in electrical schematic form even though the structure shown 45 between transistors 25 and 26, on the left, and transistor 27, on the right, is shown as a schematic indication of the top view of a monolithic integrated circuit chip. Transistors 25 to 27 would also be fabricated in the monolithic integrated circuit chip in practice. They could have also been shown in the chip top view, but are shown in electrical schematic form for ease of understanding of the system of Figure 4.

Further shown in Figure 4 is a doped polycrystalline silicon reset gate, 30, immediately adjacent the photocletectors 1 through N as interconnected by electrode 14. On the side of reset gate 30 opposite that along which photodetectors 1 through N occur is shown a series of diffused regions, 31, occurrinq in the semiconductor material body that is below and supporting reset gate 30 through an insulating layer of silicon dioxide. Diffused regions 31 are shown by dashed lines. Each of these regions 31 together with gate 30 form effectively in operation an IGFET at each photodetector site, these transistors all having a commonly 55 connected gate region as provided by gate 30. The diffused regions 31 are all electrically connected to a voltage supply, Vsupp, and gate region 30 is connected to the same operating voltage source (1), as is the gate of transistor 25.

This arrangement permits electrode 14 and the effective capacitances connected thereto to be charged to the voltage VREF while simultaneously removing any radiation induced charge that may have accumulated in 60 the CIS photocletectors at sites 1 through N prior to or during the application Of VREF. This removal is accomplished by transferring such charge from each of the photodetector sites 1 through N under gate 30 to the corresponding one of regions 31 and thence to the voltage supply supplying voltage Vsupp.

Again, transistor 26 has an operating voltage (Pd applied thereto to permit electrically connecting electrode 14to ground during times when there is the desire to introduce no additional noise into the major signal 1 7 GB 2 070 332 A 7 transfer shift register. 33,from photodetectors 1 through N via input transfer shift registers, 34. This might be desired, for instance, in the situation where main shift register 33 is arranged to have a further array of photodetectors transfer charge packets thereto for readout at times alternative to the times of transfers from the photodetectors interconnected by electrode 14.

As shown in Figure 4, main signal shift register 33 has been provided as a three phase charge-coupled device arrangement. Typically, this will be a surface channel charge- coupled device but it could also be a buried channel charge-coupled device. In any event, three electrodes in shift register 33 are shown associated with each photodetector site and its corresponding input shift register 34, these electrodes being of doped polycrystalline silicon.

Input shift registers 34 are separated and electrically isolated from one another by channel stop regions, 10 35, which also each isolate portions in main signal shift register 33 from the adjacent input shift registers 34.

Of course, there are also other channel stop regions occurring around shift registers 33 and 34 but these will not be indicated further. These channel stop regions are provided by doping regions nearthe major surface of the semiconductor material to have a p' - type conductivity at the locations desired for such stops.

Input shift registers 34 are provided by three doped polycrystalline silicon electrodes common to each 15 input shift register to N form three phase, single stage shift register. A fourth electrode is also provided, common to each input shift register 34, serving as a transfer gate to direct the transfer of charge accumulated in photodetectors 1 through N to each corresponding input shift register 34, and ultimately into main signal shift register 33 for readout of these charge packets.

In operation, a charge packet from each of the photodetectors 1 through N is simultaneously transferred 20 into its corresponding input shift register during a frame, and then each packet is simultaneously transferred into main signal shift register 33. The charge packets in main signal shift register 33 are transferred to the right during the frame and reach the resettable floating gate output arrangment provided at the end of main signal shift register 33.

In this output arrangement, a floating gate, 36, is connected to the gate of an output IGFIET, 37, operated as 25 a source follower driving a current load means, 38, connected between the source of transistor 37 and ground. Charge packets, transferring to the right in main signal shift register 33 as indicated above, pass under floating gate 33 as indicated above, leading to a voltage, VSAMp, being available across current load means 38 at the source of transistor 37.

The charge packets continue to shift along in main signal shift register 33 to reach a diffusion, 39, shown in 30 dashed lines at the end of this shift register. This diffusion is connected to the supply voltage Vsupp through which the charge packets are dissipated. Further components in the resettable floating gate output arrangement are another IGFIET, 40, and the capacitance, 41. The operation of all of these components at the end of main signal shift register 33 in the resettable floating gate output arrangement are well known and will not be further discussed here.

The remaining portions of the system shown in Figure 4 are present for generating a pulse q)Tfor operating the transfer gate common to all of the input shift registers 34. Thus, PT signals the end of the taking of a sample and provides for the transfer of those charge packets accumulated in this sampling under each of the photodetectors 1 through N. To generate OT, voltage V. at the output of 1GFIET 27 is supplied to a determination means, 42. Determination means 42 provides a determination of when voltage V. has 40 decayed sufficiently far, i.e. reached the horizontal dashed line as shown in Figure 3, to thereby indicate that the average charge packet in photodetectors 1 through N has grown sufficiently large such that sampling in a particularframe is to be terminated. As indicated within the box 42, the determination means can, in many instances, be as simple as providing a comparator having one side thereof connected to a reference voltage representing the desired voltage along the horizontal line in Figure 3 while the other input side of the 45 comparator simultaneously receives voltage V, The output of the determination means will be a voltage level shift applied to a synchronization logic means, 43, which reacts by providing a pulse in the OT waveform which is synchronized with the waveform pulses operating the shift registers, 1)1, 02, and (P3. This synchronization is necessary to coordinate the transfer of charge from each of the CIS photodetectors with the shifting sequence in the input shift registers 50 34 so that the charge packets are properly transferred from the photodetectors into these input shift registers.

The operation of the system of Figure 4 can be seen in summary in the waveforms presented in Figure 5.

The first three waveforms represent the shift register operating voltages, (p,, (P2, and %. The next waveform down in Figure 5 represents the control voltage waveform (Pr which is applied to the gate of MOSFET 25 thereby directing that the CIS photodetectors interconnected by electrode 14 be charged to the voltage value VREF. The first vertical dashed line in Figure 5 indicates where the taking of a sample begins coinciding with the ending of the Or pulse controlling transistor 25 and which begins a frame. From this first vertical dashed line in Figure 5 on, voltage V,, at the output of transfer 27 beings to decay from the value VREF - VTHRESH-27 as average charge packet in the photodetectors 1 through N begins to increase in response to the electromagnetic radiation impinging on the photodetectors.

At some point voltage V,, will have decreased sufficiently such that determination means 42 will provide a rising voltage to sychronization logic means 43. As a result, a pulse in waveform q)T is provided when the next pulse in waveform q), occurs, as can be seen in Figure 5, to begin the transfer of charge packets accumulated in photodetectors 1 through N into the corresponding input shift registers 34. Thereafter, (P, 65 8 GB 2 070 332 A 8 again returns to a high state to direct transistor 25 to provide voltage VREF to electrode 14, and to the photodetectors interconnected thereby, to begin another frame. Thus, there is provided a method for sensing the average amount of the charge packets accumulating in the interconnected photodetectors due to impinging electromagnetic radiation at the very time these charges are accumulating. Hence, no additional time in a frame need be allowed for various sensing and signal processing manipulations to determine what the average charge packet size is after the sampling period is over. Nor is there any need for providing additional photodetectors outside the interconnected array for the purpose of making a determination of charge packet size occurring by virtue of the accumulating of charge packets in the array due to impinging radiation. 10 Note that the remaining circuit components in Figure 4 can also be integrated in the same monolithic integrated circuit chip in which main signal shift register 33 and input shift register 34 are fabricated. That is, the entire device can be, as indicated above, conveniently provided in the same monolithic integrated circuit chip as the photodetectors themselves are provided in. However, in some situations, there may be advantages to not including all the device shown in Figure 4 in the same chip. 15 Several such photodetector arrays along several corresponding common electrodes, and the associated shift registers, can be provided to form an extended two-dimensional array in a monolithic integrated circuit chip if desired. Further, more than one such photodetector array on a chip can be served by a single main signal shift register with each array having input shift registers leading to such a main shift register.

Claims

1. An electromagnetic radiation sensor and processor device having a plurality of sensing sites, said device comprising a semiconductor material body of a first conductivity type except in selected regions thereof, a plurality of input charge-transfer devices each of which has therein a storage well electrode, and each said input charge-transfer device storage well electrode being spaced apart from a major surface of said body by an electrical insulating layer; a sensor electrode having therein a plurality of separated regions each region being immediately adjacent to a respective one of said storage well electrodes such that transfers of electrical charge can be directed therebetween, said electrode being space apart from said major surface by an electrical insulating layer, said first sensor electrode regions being located at at least some of said electromagnetic radiation sensing sites; switching means having first and second terminating regions and having a control region therein by which said first switching means is capable of being directed to effectively provide a conductive path of a selected conductivity between said first and second terminating regions, and first terminating region being electrically connected to a first terminal electrically connectable to a first electrical energization means, said second terminating region being electrically connected to said first sensor electrode; and a buffer having a high impedance input electrically connected to said first sensor 35 electrode, said buffer being capable of providing a representation at an output thereof of any voltage occurring at said input thereof whereby an indication of voltage occurring on said first sensor electrode can be provided in response to selected electromagnetic radiation impingement occurring therethrough at said electromagnetic radiation sensing sites.

2. The device of Claim 1, wherein the areas of said sensor electrode between said regions are spaced further from said major surface than are said regions.

3. The device of Claim 1 or 2, wherein said inut charge-transfer devices are each surface channel charge-coupled devices.

4. The device of Claim 1, 2 or 3, wherein said sensor electrode, on a surface thereof opposite that surface closest to said semiconductor material body, is at least partially covered with a blocking material capable of 45 preventing electromagnetic radiation from reaching said sensor electrode, said blocking material having openings therein exposing said electrode regions to any impinging electromagnetic radiation.

5. The device of anyone of the preceding claims, wherein said switching means and said buffer are each enhancement mode, insulated-gate field-effect transistors, said first and second terminating regions of the switching means being drain and source regions, respectively, and said control region being a gate region, said buffer input being a gate region.

6. The device of anyone of the preceding claims, wherein each input charge transfer devices has one storage well immediately adjacent to at least one other such that transfers of electrical charge can be directed therebetween, but being immediately adjacent to a maximum of two other such storage well electrodes, said input charge-transfer devices loading to a common charge- transfer device shift register directed to receive electrical charge from said input charge-transfer devices and shifting any received electrical charge therealong.

7. The device of Claim 2 or anyone of Claims 3to 6 as appendantto Claim 2, wherein said semiconductor material body is doped in larger concentrations near said major surface at locations adjacent said sensor electrode areas than is said semiconductor material body near said first major surface at locations adjacent 60 said sensor electrode regions.

8. The device of anyone of the preceding claims, wherein said semiconductor material body is of doped silicon, said sensor electrode and each of said storage well electrodes are of doped polycrystalline silicon, said insulating layers being a common insulating layer of silicon dioxide.

9. The device of Claim 8 as appendantto Claim 5, wherein the silicon dioxide layer also provides an oxide 65 0 v 9 GB 2 070 332 A 9 layer between said gate regions and said semiconductor material body for said insulated-gatefield-effect transistors serving as said switching means and as said buffer with said switching means source and drain and said buffer means source being formed in selected regions of said semiconductor material body which are of a second conductivity type.

10. The device of Claim 5 or Claim 6,7,8 or 9 as appenclaritto Claim 5, wherein said buffer output is a source region in said field-effect transistor serving as said buffer.

11. A method for sensing electromagnetic radiation for deriving signals representative of the sensed radiation, the sensing device having a semiconductor material body of a first conductivity type except in selected regions thereof with said semiconductor material body having a major surface, said device having a sensor electrode adjacent said major surface at least at a plurality of electromagnetic radiation sensing sites, 10 said device also having a plurality of detector signal transfers devices arrayed along said sensor electrode such that there is a detector signal transfer device immediately adjacent to each of said plurality of electromagnetic radiation sites, said method comprising providing, for a first selected duration, a voltage of a selected value between said sensor electrode and at least that portion of said semiconductor material body at said plurality of electromagnetic radiation sites and subsequently, for a selected second duration, eliminating any significant conductive paths connected to said sensor electrode; monitoring those values of voltage occurring between said electrode sensor and said semiconductor material body during said second selected duration while said electromagnetic radiation is impinging on said sensing sites; and in sequence, repeating each of said preceding steps a selected number of times beginning with providing a voltage of said selected value between said sensor electrode and said semiconductor material body for a said first duration 20 after that immediately preceding said second duration is past.

12. The method of Claim 11, wherein said monitoring further comprises determining when there has been a sufficient change in value of that voltage occurring on said electrode sensor during said selected second duration and providing a determination signal on that occurence to indicate termination of said second selected duration; and transferring charge from said sensing sites accumulated because of impinging electromagnetic radiation thereon to said detector signal transfer devices after said determination signal occurs.

13. The method of Claim 11 or 12, wherein said sensor electrode is part of a conductor-insulator semiconductor (CIS) photocletector present at each of said radiation sensing sites.

14. The method of Claim 12 or 13 as appendaritto Claim 12, wherein said transferring of charge is 30 accomplished by a charge-coupled device leading from each of said sensing sites.

15. The method of anyone of Claims 11 to 14, wherein said providing of a voltage of a selected value is accomplished by use of an enhancement mode, insulated-gate field-effect transistor connected between said sensor electrode and an inter-connection electrically connected to an electrical energization source, and said monitoring of said electrode sensor is accomplished at least in part by an enhancement mode, insulated-gate field-effect transistor having a gate region thereof connected to said sensor electrode.

16. The method of Claim 14 or Claim 15 as appendant to Claim 14, wherein said charge-cou pled devices each lead to a common charge-coupled device shift register which receives charge from said charge-cou pled devices leading from said sensing sites.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1981. Published by The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.